Recently, in large part due to escalating gasoline prices, there has been an intensified interest in components, systems and apparatuses that are able to increase fuel efficiency in motor vehicles. One area of interest has been the production of more fuel-efficient engines and unique drive trains, which allow these more fuel-efficient engines to generally operate over a narrower band of rotational speeds. Recently, major automobile manufacturers such as Nissan and Toyota have offered vehicles with Continuously Variable Transmissions (CVTs) and gas/electric hybrid drive trains. Both such systems allow the internal combustion engine powering the vehicle to operate over relatively narrow ranges, which in turn allows engine designers to design those engines for maximum efficiency within those relatively narrow ranges.
A significant operational requirement of an automotive drive train is typically to be operable to connect and disconnect an internal combustion engine from the rest of the drive train. In vehicles with typical manual transmissions, this function is performed by a manual clutch. When a vehicle operator depresses a clutch pedal, a pressure plate is moved away from a clutch disc and flywheel, thereby disconnecting the drive train from the internal combustion engine. When the clutch pedal is released, the pressure plate moves toward the flywheel, sandwiching the clutch between the pressure plate and the flywheel. Friction between the pressure plate, clutch, and flywheel eventually brings the components together in common rotation. However, between the disconnected state and the common rotation state, the components may be in contact but not rotating at a common speed. During this period of slippage, heat is generated. The generated heat is unrecoverable energy loss. In an automatic transmission, the function of connecting and disconnecting the internal combustion engine is performed by a torque converter. The torque converter uses a fluid coupling as the disconnect mechanism. The torque converter also allows slippage and heat is generated within the fluid of the torque converter. Typically, this fluid is cooled through a transmission fluid cooler, and again, the heat generated is unrecovered energy loss.
Typical internal combustion engines are designed to operate over a relatively large range of rotational speeds. This is to enable the engine to power the vehicle in a wide variety of conditions, which include starting from a standing stop, accelerating at various rates, and high-speed cruising. However, internal combustion engines can be made more efficient if they are designed to operate over a narrow band of rotational speeds. Hence, design trade-offs have typically been made between efficiency over a narrow range of rotational speeds and adequate power over a wide range of rotational speeds.
To ease the impact of the aforementioned internal combustion engine design trade-offs, automotive designers have increased the number of gear ratios present in the typical automotive vehicle. Today, five-speed automatic transmissions and six-speed manual transmissions are not uncommon. However, these transmissions are more complex and require more shifting to maintain the internal combustion engines within their optimal ranges of rotational speeds.
To further control the range of rotational speeds at which internal combustion engines must typically operate, many automotive manufacturers offer CVTs. These CVTs may, under some circumstances, allow the internal combustion engine to operate at a particular rotational speed while the transmission continuously varies its gear ratio to accommodate varying vehicle speeds. This allows the internal combustion engines to operate over an even narrower range of rotational speeds, which in turn allows engine designers to optimize the engine for operation within that narrower range.
Recently, gas/electric hybrid vehicles, such as Toyota's Prius®, have been offered for sale. One advantage of a gas/electric hybrid vehicle is that the internal combustion engine may be operated at very specific rotational speeds, and therefore engine efficiency can be maximized for operation at those rotational speeds. By adding an electrical energy storage device such as a battery, gas/electric hybrids are also able to recapture some of the kinetic energy of the vehicle by operating the electric motors as generators during braking, which generates electricity that is then stored in the battery. This is known as regenerative braking, and the stored energy is available to power the vehicle under certain conditions. The battery also may supply power during peak demand periods, such as hard acceleration. This in turn allows the internal combustion engine to be relatively smaller than what would typically be required for the size of vehicle in which it is installed since its peak power output will be supplemented by energy from the battery. This in turn leads to even greater fuel efficiency.
Gas/electric hybrid systems are generally more expensive than traditional internal combustion engine drive trains. The batteries used in gas/electric hybrid systems are generally expensive and heavy. There are also safety concerns with the electrical systems in these vehicles; some of which operate at 500 V. Gas/electric hybrid technology is generally only available in vehicles specifically designed to take advantage of the technology, which in general means that only new vehicles feature the technology and retrofitting existing vehicles may be relatively expensive.